The invention relates to an asynchronous transfer mode method of transmitting digital data made up of cells (packets), in which method terminals send messages to a central station (control station).
It relates more particularly, but not exclusively, to a transmission method for use in a system in which calls are relayed via equipment on board a satellite in a non-geosynchronous orbit.
For optimum use of a telecommunication system it is preferable to manage the information transmitted so that at all times information can be transmitted at a bit rate equal to the maximum bit rate allowed by the system.
To this end, the information is transmitted in digital form, to limit noise and facilitate control. The digital information is usually divided into cells (packets) which can be transmitted during a given time interval—referred to as the cell interval—and the cells are transmitted with a time distribution that optimizes system use. In other words, the cells are not transmitted regularly, but in a manner that is sometimes referred to as asynchronous transfer mode; note that this does not limit the invention to the ATM standard, however.
What is more, to maximize call capacities, each cell (packet) can be allocated a carrier frequency and/or a code selected from a multiplicity of frequencies and codes.
There are three ways to divide radio resources (communication resources):                Time Division Multiple Access (TDMA).        Frequency Division Multiple Access (FDMA).        Code Division Multiple Access (CDMA).        
Assigning a code to a signal spreads its spectrum, i.e. multiplies the signal by a spreading code. Orthogonal codes, i.e. codes such that the product of a code multiplied by itself is equal to 1 and the product of two different codes is zero, are used for easy and powerful decoding (despreading). Then, when signals x1, x2, . . . xi, . . . xn are transmitted simultaneously, and each is assigned a respective code C1, C2, Ci, Cn, all that is required to extract the signal xi from the sum x1C1+x2C2+ . . . +xiCi+ . . . +xnCn is to multiply the sum by Ci.
U.S. Pat. No. 5,373,502 describes a transmission method in which terminals send cells to a station, the terminals send successively in separate periods, and each cell is assigned at least two orthogonal codes. This transmission technique is known as TD-CDMA.